I've never found anything like that on the internet. Sometimes it could be a last minute booking for an act so even the cruise line doesn't always know until a few days before the cruise. My experience with Carnival - at least on cruises leaving from U.S. ports - is they have always had a comedian, a country band, a rock band, a jazz group, and "island" musicians for Caribbean cruises. Sometimes the island group also did the rock and roll.
We have a couple forum members with inside connections at Carnival, and they often know the entertainment schedules for their cruises beforehand.